FIG. 1 of the accompanying drawings illustrates, schematically, a fuel injection system of the common-rail type for use in an internal combustion engine, for example as described in EP-A-1921307. An accumulator volume for fuel, known as a common rail 10, is supplied with high pressure fuel from a high-pressure fuel pump 12. The high-pressure pump 12 includes a pump chamber 12a which receives fuel from a low-pressure source or reservoir 14 by way of a metering valve 16. The pump 12 also includes a pumping element or plunger 12b which is driven in linear reciprocal motion to change the volume of the pump chamber 12a in a cyclical manner. On a filling or return stroke of the plunger 12b, fuel is drawn from the reservoir 14 into the pump chamber 12a, and on a pumping or forward stroke of the plunger 12b, fuel is pressurised in the pump chamber 12a and is forced under high pressure into the common rail 10.
The common rail 10 supplies high-pressure fuel to a plurality of fuel injectors 18, only one of which is shown in FIG. 1, and each fuel injector is operable under the control of a control valve 20 to cause injection of fuel into an associated cylinder 22 of the engine.
To maintain the high fuel pressure in the common rail 10, and to prevent high-pressure fuel from the common rail 10 flowing back towards the pump 12 and the metering valve 16, particularly during the return stroke of the plunger 12b, it is necessary to include a non-return valve 24 (also known as a one-way valve or a check valve) in the fuel flow path between the pump 12 and the common rail 10.
The non-return valve 24 comprises a ball 24a received in a valve chamber 24b. An inlet passage 26, which is in fluid communication with the pump 12, opens into the valve chamber at a valve seat 24c. An outlet passage 28 opens into the valve chamber at a location remote from the valve seat 24c, such that fluid communication between the valve chamber 24b and the outlet passage 28 is continually open. The outlet passage 28 is in fluid communication with the common rail 10.
The ball 24a is biased towards the valve seat 24c by a valve spring 24d. During the forward stroke of the plunger 12b, the ball 24a moves away from the valve seat 24c to allow fuel to flow from the inlet passage 26, through the valve chamber 24b, and to the common rail 10 through the outlet passage 28. During the return stroke of the plunger 12b, the ball 24a is caused to engage with the valve seat 24c by the spring 24d. Flow from the common rail 10 back to the pump chamber 12a is thereby prevented. In this arrangement, movement of the ball 24a can be precisely controlled by suitable selection of the spring 24d, which determines the force with which the ball 24a is urged towards the valve seat 24c. 
To reduce the part count of the valve and to improve reliability, it can be desirable to omit the valve spring 24d, so that the ball 24c is free to move within the valve chamber 24b. In such an arrangement, during the return stroke of the plunger 12b, the ball 24a is drawn to engage with the valve seat 24c by the partial vacuum created by the volume increase in the pump chamber 12a, and is kept in contact with the valve seat 24c by the high rail pressure acting on the ball 24a. Flow from the common rail 10 back to the pump chamber 12a is thereby prevented. However, since the forces acting on the ball in this arrangement are exclusively derived from the fuel pressure acting on each side of the ball 24a, movement of the ball 24a is less well-controlled, and therefore the opening and closing behaviour of the non-return valve is less predictable.
Against this background, it would be desirable to provide a non-return valve arrangement with good reliability, low part count and precisely-defined opening and closing behaviour.